1. Field of the Invention
The present invention relates to a communication method and system suitable for radio link systems, and more particularly to a communication method and system using a multiple access control wherein access rights to transmit data from a plurality of mobile stations (substations) are managed by a base station (control station).
2. Description of the Related Art
A polling method and a pre-assigned Time Division Multiple Access (TDMA) method are known examples of a multiple access control method in which a control station or base station collectively controls a plurality of terminal equipment to allow them to access a communication medium (communication channel) and transmit data.
In the polling method, a base station inquires of each terminal equipment (hereinafter called a substation) whether it has any data to transmit. Therefore, there is no possibility that a plurality of will transmit data at the same time and cause the data to collide on a communication medium, and this method has an advantage that an access right can be assigned to each substation equally. However, a large bandwidth of the communication medium is occupied by polling messages from the base station to the substations.
In the pre-assigned TDMA method, the base station pre-assigns an access time for accessing a communication to each substation and each substation periodically transmits data at the access time assigned to it. There is no possibility that data from a plurality of substations will collide on the medium, and there is no need of transmitting polling messages from the base station the substations. This method therefore simplifies the multiple access control.
With the TDMA method, however, the access time is assigned even if a substation does not need to transmit data. Therefore, if this method is applied to a system such as a Local Area Network (LAN) in which bursty data transmission occurs between terminal equipment, the efficiency of use of the communication medium is degraded. Especially in a wireless LAN in which mobile stations are used as substations, registration/removal of a mobile station to/from a communication area (cell) occurs frequently due to the movement of the mobile station. It is necessary for a control station to assign a new bandwidth to the mobile station, degrading the efficiency of use of the communication medium.
As a conventional technique offering a solution to these problems, there is known a split-channel reservation multiple access method. With this method, a communication frame is split into a control data transmission field and a message transmission field. The control data transmission field is further divided into a transmission request field having a plurality of slots, and reply fields corresponding to respective ones of the slots. When a substation requests an access right to a base station by using one of the slots in the transmission request field, the base station uses the request field corresponding to the slot to notify the substation of a usable field in the message transmission field. An example of the split-channel reservation multiple access method is described, for example, in "Wireless In-Building Network Architecture and Protocols", IEEE Network Magazine, November 1991, pp. 31-38 (hereinafter, this method is called conventional method 1).
According to the conventional method 1, at each substation assigned an access right by the base station, information (a message) to be transmitted is divided into a plurality of information blocks each having a fixed length, and each information block is transmitted in a fixed length packet field (fragment) called a "fragment slot" defined in the message transmission field.
Information blocks transmitted in fragments are received by the base station which in turn transmits the information blocks in fragment slots of another communication frame and transmits them to a destination substation. If the information block transmission to the destination station fails, the source substation retransmits the same information blocks.
The message transmission field has a plurality of fragment slots. Each fragment slot is constituted by an information field in which an information block is set, a block number field in which stored is the block number indicating the location of an information block in one message, and a code field in which an error correcting/detecting code is set.
According to the conventional method 1, a substation requests data transmission in units of fragment by a slotted ALOHA method, using one of the request slots defined in the control information transmission field. If a collision occurs because another substation uses the same request slot, these substations use another communication frame after the collision frame and request again data transmission by using a randomly selected request slot of the frame.
In a conventional success/failure reply relative to a message transmitted from a source station to a destination station, a reply method has been used in which an acknowledge (ACK) pattern is sent back for a reception success and a non-acknowledge (NAK) pattern is sent back for a reception failure.
This method is, however, associated with a problem regarding the broadcast communication for transmitting the same message to a plurality of substations. For example, in a ring-LAN, the NAK pattern sent from a LAN node may be changed to the ACK pattern at another downstream LAN node. In an Ethernet (IEEE 802.3 LAN) or wireless LAN in which a single communication medium is shared by a plurality of substations, if reply signals are sent from a plurality of destination substations, a collision occurs on the communication medium so that ACK and NAK patterns set in the reply fields may be changed.
An example of a conventional technique solving these problems is described, for example, in "A Multicast ARQ Scheme for the Vehicular Communications", the 13th Symposium on Information Theory and Its Applications, pp. 623-626 (hereinafter, this method is called a conventional method 2).
According to the conventional method 2, a NAK signal is sent back from each substation only when the reception of a broadcast message is failed, and a broad reply field is used to allow the reception of replies and reduce a collision probability of NAK signals. In this fashion, even if a collision occurs, the same frame can be used to send back again the reply signal.
It is known that the transmission power of an antenna in a wireless network attenuates in inverse proportion to a square of a distance ratio between transmission and reception stations. Even if the same transmitting power is used in transmitting a signal from a plurality of substations differently remoted from the base station, each received power is sensed at the base station different for respective substations.
For example, assuming that two substations A and B located respectively at distances 1 m and 10 m from the base station transmit signals at the same transmitting power, the signal power of the substation A received at the base station is 100 times as large as that of the substation B.
If the conventional method 1 is applied to a wireless network, the following two cases of receiving transmission requests may occur depending upon the position (distance) relationship between a base station and substations, when a collision of transmission requests issued by a plurality of substations occurs on the same request slot.
In the first case, a plurality of transmission requests are mixed at substantially the same receiving power, and so all the transmission requests are judged as error signals.
In the second case, the transmission request having the maximum receiving power is correctly processed.
As the transmission power attenuates as described above, a transmission request made by a substation nearest the base station is most likely to have a transmission right over other substations, on the assumption of no shadowing.
Consider for example that two substations A and B transmit a packet of 100 bit length at the same transmitting power at the same time, and that the packets are received as signals a and b at receiving powers Sa and Sb. Assuming that the signal b is noises of the signal a, the signal a has a high possibility of being detected by the base station as a normal signal if the following relationship is satisfied: EQU Sa/(N+Sb)&lt;.alpha.
where .alpha. is an SN ratio at a bit error rate 1E-2. This situation corresponds to the second case.
The right side term of the above formula is about 9 dB in the case of Quadrature Phase Shift Keying (QPSK) using differential detection, according to the calculation described in the document "Digitalization Technology Mobile Communication", p. 77, TRICEPS in "Bit Error Rate vs. Signal to Noise Ratio Characteristics". Since Sa=r*r*Sb and Sb/N (average error rate 1E-4 at radio link)&gt;1, the distance r.gtoreq.2.8 m.
The communication coverage area of a base station in a wireless network is generally about several 10 m to several 100 m in radius. Therefore, the second case occurs easily in practical applications.
The second case is an unfair access control because a particular substation is preferentially assigned a transmission right upon occurrence of a contention between transmission requests. In the document of the conventional method 1, however, no means is presented for realizing a fair access control independent of the locations of substations.
The unfair access in the second case can be dissolved, for example, by controlling the transmitting power at each substation to be received as a constant power at the base station. According to a conventional general control method, each substation monitors the receiving power of a signal transmitted from the base station to estimate the distance to the base station and control the transmitting power at the substation. An example of this control method is described, for example, in "On the System Design Aspects of Code Division Multiple Access (CDMA) Applied to Digital Cellular and Personal Communications Networks", IEEE VTS '91, Proceedings, pp. 57-62 (hereinafter called a conventional method 3).
A method such as the conventional method 1 in which all messages from source to destination substations are transmitted always via the base station, has advantages of an availability of central communication management at the base station and moreover of a reduction of hidden terminals specific to a wireless LAN. Namely, with the method of direct communications between substations, communication is disabled if any obstacle is present between substations. However, with the method of indirect communications via a base station, if the base station and substations are located line-of-sight, the effects of an obstacle therebetween can be avoided.
With the conventional method 1, however, upward channels from a substation to a base station and downward channels from the base station to a substation are required to be separately provided in terms of frequency and time. Therefore, the transmission efficiency is lowered to a half of that of the method of direct data communications between substations, posing a problem of an inability of using transmission resources efficiently.
In a wireless LAN, the size of each fragment is generally about several hundreds bytes. The maximum length of a message transmitted by a terminal equipment (substation) in a wireless LAN is about 1.5K bytes to ten and several K bytes far greater than the fragment size. If the access request is made in units of fragment as in the case of the conventional method 1, each substation is required to execute an access request operation several to several tens times in transmitting one message. This access right request operation is executed also in transmitting again the same message. The collision probability of access right requests on the request field increases if the conventional method 1 is applied to a wireless LAN, posing a problem of lowered efficiency.
The communication frame structure of the conventional method 1 does not prepare a special field for use by a destination substation to notify a reply to the source substation indicating a reception success/failure of a transmitted fragment. As a result, the destination substation is required to obtain a fragment slot in the manner similar to the access operation to a fragment by the source station, and is required to send back the reply message. This necessity is also one of the factors degrading the efficiency and increasing a transmission wait time.
Furthermore, a delay of a reply from a destination substation leads to a lowered end-to-end throughput. As an example of a data retransmission method aiming at preventing the degradation of efficiency due to a reply delay, there is known, for example, a selective retransmission method wherein only a fragment failed in reception is retransmitted. This retransmission method is associated with a problem of a need of a complicated buffer management function at a substation.
If the length of the fragment field is designed to be short, the field occupied by the fragment header becomes large so that the size of each data block becomes small, degrading the transmission efficiency. Conversely, if it is designed to be long, a probability of data retransmission due to a failure of data transmission becomes large and the amount of retransmission data increases, degrading the transmission efficiency. There is therefore an optimum size range of a fragment length determined from the transmission efficiency of each system.
In a LAN system, the protocol higher than the logical link control (LLC) layer does not depend on the type of a transmission medium. An interface from the media access control (MAC) layer to the LLC layer is generally required to have a high quality of a bit error rate of 1E-8 or higher. Therefore, it is necessary for a wireless LAN having an average bit error rate of about 1E-4 to improve the bit error rate of 1E-4 to 1E-8 or more at the layers lower than the MAC layer.
In such a case, Hamming codes or BCH codes for example are used. Taking into consideration the coding efficiency and an original bit error rate of about 1E-2 under a hidden terminal environment, it is necessary to use several tens bytes for the correction block. Accordingly, in the frame structure of the conventional method 1 wherein the fragment structure has only one error correcting and detecting field, the fragment length is limited by the size of the correction block. The actual fragment length becomes much shorter than the optimum fragment length calculated based on the error block retransmission and header overhead. It is therefore difficult to gain a maximum efficiency if this fragment structure is used.
Another factor affecting the communication efficiency in the split-channel reservation multiple access method is a success rate at the request field. As the number of request slots of the request field of each communication frame is increased, an apparent access request success rate is improved. However, the size of the message transmission field (information field) becomes small as the size of the request field becomes large, being unable to improve the substantial communication efficiency.
In an actual system, the communication efficiency depends largely upon the access request retransmission procedure, i.e., backoff algorithm, to be executed when a plurality of access requests collide on the same request slot. With the conventional method 1, however, while an access request retransmission operation upon a collision is repeated, a new access request may be issued from another substation, increasing a collision probability and resulting in congestion of the system.
For the conventional method 2 regarding the reply to the reception of broadcast communication, it is impossible to make a collision probability of NAKs zero and provide an essential solution to the collision problem. With this method 2, if the size of the communication frame is limited, the message transmission field is required to be made narrow as the reply field is broadened, lowering the efficiency.
In an in-door wireless communication environment, transmitted radio waves are reflected by a wall or the like so that the same signal reaches a reception station via different paths. The transmission path is affected by the opening/closing of doors, sway of blinds or curtains, motion of people, or other obstacles. Therefore, radio waves change their phases to make the amplitudes smaller or larger. The Doppler frequency is about several tens Hz in this case, and the frequency of communication frames are generally several tens Hz. As a result, even if a substation tries to control its transmitting power in accordance with the receiving power from the base station, it is difficult to precisely determine the transmitting power because a difference between an estimated time and a transmission time is in the order of the fading period.
An unfair problem of the access control caused by a difference between receiving powers in a wireless network is not essentially unfair, when compared to an access control method with respect to a collision in ALOHA, slotted ALOHA, CSMA, or CSMA/CD in a wired network environment. Specifically, in the wired network environment, if an access request collides, the communication fails by all means. In the wireless network environment, if an access request collides with another access request from another substation, the communication fails in one case and succeeds in another case at the substation nearer to the base station. From this viewpoint, it can be said that the communication efficiency is better. The main issue of the communication unfair problem resides not in that there is a surviving substation, but in that the surviving substation is limited to the substation nearest the base station.
Although the conventional method 3 can deal with the unfair access, it is practically impossible to apply this method to a wireless network to correctly control the transmitting power at a substation. Even if the transmitting power could be controlled correctly, the above-described success chance is lost, resulting in a lower communication efficiency. In this context, this method 3 is not always optimum.
For a local network in which one of a plurality of substations which obtained a transmission right is allowed to transmit data, there is known a control method in which a transmission right priority can be assigned to a particular substation. For example, in a Token Ring IEEE 802.5, a priority bit is assigned to each packet. CSMA/CD with priority is also presented, for example, in the proceedings on IEICE Conference '81, 1-276 and in the Journal of Information Processing Society of Japan, Vol. 23, No. 12, '82, pp. 1139-1140.
However, such priority control becomes a complicated control at the MAC layer level. Such priority control is not suitable for the access control method for a network wherein a collision detection function is not provided to each substation, and wherein CSMA, slotted ALOPHA or the like is used assuming a collision between a plurality of access requests.